I'll be the first to admit that this isn't a scientific study, but I've seen an alarming number of older lifelong competitive athletes with health problems. A-fib, other heart rhythm issues, enlarged rupture-prone aortas, calcified arteries, and so on.
On the cosmetic side, a large number of them look way older than their actual age and have leathery, wrinkled faces and hands. On the performance side, 2+ decades of intense training wears down your muscles, joints, and tendons. The best masters runners are often those who didn't train much in their youth and still have fresh legs, and the exceptions to this like Lagat usually take long extended breaks after each season.
Those issues aren't exclusive to runners either. If anything, the cardiovascular issues among long-time cyclists and XC skiers are even more common. The one common trend is that prolonged endurance training isn't good for your health and isn't worth it from a performance standpoint.
When you're young, perhaps from your early/mid teens through your early/mid 30s, go ahead and put in those miles. Chase those victories and PRs, enjoy the time with your college teammates, and dream of making the Olympics or Olympic Trials. But after that, hang up the spikes and just jog or hike to stay fit. A few sub-par late-career years aren't worth a heart valve replacement.